Residential and urban development in the context of flood plain rivers and hydrological and hydraulic conditions regardless of the watershed and river, an increasment risk of flooding and loss of the other side can make investment in these areas. The purpose of this  study is to evaluate the behavior of river flooding and the spread of Karaj in Alborz province. combining hydrological model HEC-HMS and HEC-RAS hydraulic model was used for this purpose, meteorological and hydrometric stations using rainfall data and runoff area HEC-HMS model was calibrated and river cross sections taken at intervals residential areas, flood zones showed with return periods of 10, 20 and 50 years in GIS using HEC-GeoRAS amendments in Arc view is displayed.